In a UMTS (Universal Mobile Telecommunications System) network, long-term evolution (LTE) is under study for the purposes of further increasing high-speed data rates, providing low delay, and so on (non-patent literature 1). In LTE, as multiple access schemes, a scheme that is based on OFDMA (Orthogonal Frequency Division Multiple Access) is used on the downlink, and a scheme that is based on SC-FDMA (Single Carrier Frequency Division Multiple Access) is used on the uplink.
In the LTE system, uplink signals are mapped to adequate radio resources and transmitted from a user terminal to a radio base station apparatus. To be more specific, uplink user data is transmitted using an uplink shared channel (PUSCH: Physical Uplink Shared Channel). Also, uplink control information (UCI) is transmitted by using the PUSCH when transmitted with uplink user data or by using an uplink control channel (PUCCH: Physical Uplink Control Channel) when transmitted alone.
Uplink control information (UCI) includes delivery acknowledgment (ACK/NACK) for a downlink shared channel (PDSCH: Physical Downlink Shared Channel), a scheduling request, channel state information (CSI), and so on (see, for example, non-patent literature 2). Channel state information (hereinafter referred to as “CSI”) is information that is based on the dynamic channel state on the downlink, and may be, for example, channel quality information (CQIs), precoding matrix indicators (PMIs), rank indicators (RIs) and so on. This CSI is reported from a user terminal to a radio base station apparatus periodically or aperiodically.
In response to a trigger from a radio base station apparatus, aperiodic channel state information (aperiodic CSI) is reported from a user terminal to that radio base station. This trigger (aperiodic CSI triggering) is included in an uplink scheduling grant (hereinafter referred to as “UL (Uplink) grant”) (DCI format 0/4) that is transmitted in a downlink control channel (PDCCH: Physical Downlink Control Channel). Following the trigger included in this UL grant, the user terminal reports aperiodic channel state information (hereinafter referred to as “A-CSI”) using the PUSCH indicated by this UL grant. This reporting of A-CSI is also referred to as “aperiodic channel state information reporting (aperiodic CSI (CQI/PMI/RI) reporting).”